No pain maximum gain: Carriers have myriad technologies to help ease the spectrum crunch

Telecommunications, June, 2002 by Sean Buckley

In mobile wireless, there's one inescapable truth: spectrum is finite. Couple that with a shift in Wall Street's expectations and ongoing growth in minutes of use in voice and data, and the drive to maximize existing infrastructure is essential.

Pressure continues to mount as operators begin offering data services, a market the Strategis Group projects will have a penetration rate of nearly 60 percent by 2007. AT&T's launch of GSM/GPRS data services and partnership with Cingular to build an overlay GSM network atop its TDMA network will likely be a big mover. CDMA carriers like Verizon and Bell Canada have launched their 1XRTT data networks, and Sprint PCS is not far behind.

Even moves like the FCC's spectrum cap lift from 45 MHz to 55 MHz are a quick fix; data services will eat up spectrum quickly. While building more base stations is the easy answer, the realities of cost and zoning issues are forcing carriers to examine new alternatives.

"There seems to be this bright and glimmering mirage of new spectrum that always seems to go away," said Metawave's Dr. Marty Feuerstein, senior vice president and general manager of product development. "Carriers are being forced to be more efficient with their current spectrum as traditional ways of adding capacity diminish."

As a remedy, smart antennas, high-gain amplifiers, optimization frequency planning, and cryogenic filtering will enable carriers to not only avoid deploying new base stations, but also improve the quality of existing networks.

Shaping the Beam

For many, smart antennas are seen as mobile wireless' final frontier, With modulation and coding reaching their limits, smart antennas can be used for load balancing, frequency reuse and cochannel interference. They can be bolted to existing base stations or come standard in new ones. There are two approaches: adaptive beamforming and its various implementations, and MIMO (multiple input, multiple output).

Solutions need to integrate with existing towers and double capacity cost effectively. "The functional requirements and the cost benefit have to be there," said Tom Crook, director of research and development for Sprint PCS. "Granted, spectrum is a valuable resource, but you can't pay any price for any solution."

To gain market traction, vendors have sought OEM partnerships in the United States and abroad. Metawave, for example, reached to Asia via Samsung to develop an embedded, adaptive array ASIC for 2.5G- and 3G-IXRTT and WCDMA. Metawave's Spotlight series can meet the needs of CDMA, GSM and 3G networks. For CDMA, it offers the Spotlight 2200 for traffic balancing and to reduce handoff overhead; it's also an integrated solution for Lucent's Flexent base stations. Its Spotlight GSM product integrates with existing base stations to improve capacity by mitigating cochannel interference. In July, Metawave will supply Verizon its three-sector antenna CDMA solution via its existing relationship with GTE, AirTouch and Bell Atlantic,

ArrayComm has a presence in Asia through Kyocera, with whom it helped facilitate China Telecom's deployment with Lucent and ZTE. It also has relationships with Marconi to integrate its technology for WCDMA and with the System on a Chip division of Telecom Italia Labs to examine smart antennas for 3G. ArrayComm's Intellicell utilizes spatial channels where each traditional channel is used multiple times in each sector, lending itself to SDMA (spatial division multiple access). A complement to TDMA and CDMA. SDMA uses the spatial element to identify every user by its unique spatial signature.

By utilizing adaptive beamforming from the base station with an antenna array that can sense how the environment looks at any given time in terms of interference sources, it comes up with a processing strategy to optimize the user signal quality. In an adaptive antenna solution, a base station with an antenna array makes measurements on the uplink environment, but the tricky part is the downlink: In most systems there's no feedback on how well the antenna is working, so it's a guessing game. This is a problem in a FDD (frequency division duplexing)based network because the uplink and downlink are separated by 50 MHz, and the environment can change rapidly over that frequency spread. To mitigate this dilemma, ArrayComm incorporates its smart antenna technology with adaptive beamforming technology and TDD (time division duplexing), which has an identical uplink. Along with its smart antenna technology, ArrayComm has been making strides abroad through partners with its iBurst data-only product as an add-on serv ice to existing wireless networks. Similarly, Wireless Online is conducting trials of its GSM/GPRS smart base station with Chinese OEM Shanghai Datang Mobile, In an earlier trial held in San Francisco, it delivered a 60-percent reduction in dropped calls with an average C/I improvement of 8.5 db and 3x improvement in coverage.

Large vendors are also developing solutions. Ericsson's GSM capacity booster, an adaptive antenna solution consisting of an indoor base station and a passive array antenna with an integrated sector antenna, splits the sector into eight fixed subsectors. Within each subsector, the capacity booster dynamically selects the best lobe to handle a call. Multibranch diversity is achieved by combining the best signals, improving quality by mitigating uplink and downlink interference. Nortel Networks has developed an adaptive antenna for MetroCell modular upgrade path called ABSS (Adaptive Antenna Beam Selection).